Glazing

R-Value & Thermal Performance

Glazing's R-value is a measure of its resistance to heat flow. While glass is a poor insulator on its own, modern glazing systems have significantly improved thermal performance through various technologies:

• Insulated Glass Units (IGUs): These units consist of two or more panes of glass separated by a sealed air or gas-filled space. The space between the panes dramatically increases the R-value compared to a single pane.

• Low-E (Low-Emissivity) Coatings: These microscopic, transparent metallic coatings are applied to a glass surface to reflect radiant heat. In winter, they reflect heat back into the building, and in summer, they reflect heat from the sun away from the building, helping to keep the interior comfortable year-round.

• Gas Fills: The space between glass panes in an IGU can be filled with an inert gas like argon or krypton. These gases are denser than air and are better insulators, further reducing heat transfer.

A single pane of glass has an R-value of about R-0.9, while a modern double-pane window with a gas fill and Low-E coatings can have an R-value of R-3.8 to R-5.0. The most advanced triple-pane windows can achieve an R-value of R-5.4 to R-10.0, but this is still significantly lower than a well-insulated wall, which can be R-15 to R-21 or more.

Air & Vapor Barrier

Glazing plays a vital role in a building's air barrier and vapor barrier.

• Air Barrier: A window's frame, sash, and the sealed glass unit itself must work together to prevent air leakage. Manufacturers design window assemblies to be air-tight to prevent drafts and reduce heat loss. The seals, gaskets, and proper installation of the window into the wall opening are all part of the continuous air barrier. Air leaks can lead to significant energy loss and can carry moisture into the wall cavity, causing condensation.

• Vapor Barrier: IGUs act as an effective vapor barrier. The sealed space between the panes prevents water vapor from moving through the window. This is especially important in cold climates where warm, moist indoor air would otherwise condense on the cold exterior glass, leading to condensation, frost, and even damage to the window components.

Importance of Glazing

Glazing is incredibly important to a building's overall performance. While windows are often the weakest thermal link in a building's envelope, their functions are irreplaceable. They provide natural light (daylighting), which reduces the need for artificial lighting, and they offer views that can improve occupant well-being. The type of glazing selected has a major impact on a building's energy consumption, thermal comfort, and durability. Choosing high-performance glazing is a critical decision in modern construction and a key part of creating a truly energy-efficient and sustainable building.

Double or triple-pane windows are filled with inert, low-conductivity gases for a reason. These gases are denser and move more slowly than the nitrogen and oxygen molecules in air. This density reduces heat transfer through convection within the sealed airspace.

• Argon Gas: This is the most common gas used. It's about 38% denser than air, which significantly slows down convective heat transfer. A standard double-pane window with an air-filled gap has an R-value of about R-2. With argon gas and a Low-E coating, that R-value can increase to R-3.8 or more.

• Krypton Gas: This gas is even denser and is often used in triple-pane windows with narrower gaps between the panes. It provides an even higher R-value than argon, but at a greater cost.

When the seal on the window unit fails, the gas gradually leaks out and is replaced by air. This changes the properties of the sealed space.

The Effects of a Window Gas Leak

1. Reduced R-Value: As air replaces the insulating gas, the R-value of the window drops. This increases the rate of heat loss in the winter and heat gain in the summer, leading to higher energy bills. While the exact drop depends on the initial R-value and the type of gas, a complete loss of gas can reduce a window's thermal performance by 10-20%.

2. Condensation: The most common and visible sign of a gas leak is condensation or fogging between the panes of glass. This occurs because the seal has failed, allowing moisture-laden air to enter the formerly sealed space. When the temperature of the outer pane drops below the dew point of the trapped air, moisture condenses on the inside surface of the glass.

3. Visual Distortions: In some cases, a severe leak can cause the glass panes to flex or bow inward due to a change in pressure. This can lead to visible distortions or a "rainbow" effect when viewed at an angle.

Gas-filled windows are designed to lose only about 1% of their gas per year, meaning they can remain effective for decades. However, a faulty seal or damage to the window can accelerate this process.